1. Field of the Invention
The present invention relates to an optical polymer film, a method and an apparatus for producing the a film, and more particularly to an optical polymer film with optical properties, a method and an apparatus for producing the optical polymer film, which is used as a protective film for a polarizing filter, in an optical functional film and in a liquid crystal display.
2. Description Related to the Prior Art
The liquid crystal display (hereinafter LCD) is constructed of a liquid crystal cell, a polarized element and an optical compensation sheet (birefringence sheet). In the transmittance type of the LCD, two polarized elements are provided in both sides of the liquid crystal cell, and one or two optical compensation sheet is disposed between the liquid crystal cell and the polarized element. In the reflection type of the LCD are superimposed a reflecting sheet, a liquid crystal cell, one optical compensation sheet, one polarized element in this order.
The liquid crystal cell is constructed of rod-like liquid crystal molecules, two substrates for including them, and electrode layers for applying an electric voltage to the rod-like molecules. There are several displaying mode types of the liquid crystal cell, which are used depending on situations of orientations of the liquid crystal molecules to be used. For example, as a transmitting type, there are TN (Twisted Nematic), IPS (In-Plane Switching), FLC (Ferroelectric Liquid crystal), OCB (Optically Compensatory Bend), STN (Super Twisted Nematic), VA (Vertically Aligned) and the like, and as a reflecting type, there are TN (Twisted Nematic), HAN (Hybrid Aligned Nematic) and the like. The polarizing element usually has a structure that two transparent protective films are attached to both surfaces of a polarized film.
Further, the optical compensation sheet is used in several liquid crystal displays for reducing the irregular coloring of the displayed image, or for enlarging the view angles. In the optical compensation sheet, a birefringent film which is obtained by stretching or drawing was priory used. Instead of the optical compensation sheet including the birefringent film, it is proposed in recent years to use the optical compensation sheet having optical anisotropy which liquid crystal molecules (especially discotic liquid crystal molecules) on the transparent substrate provides.
In order to provide the optical anisotropy, the orientation of liquid crystalline molecules are made and fixed. In this case, the liquid crystalline molecules have polymerizable groups, and the polymerization of the liquid crystalline molecules is performed for the fixing. The liquid crystalline molecules have large birefringence, and show several orientations. The utilization of these characters can provides the optical compensation sheet with optical properties, which could not obtain by using the prior birefringent film.
The optical properties of the optical compensation sheet are determined according to optical properties of the liquid crystal cell, concretely the above described displaying modes. Accordingly, when the liquid crystalline molecules, especially discotic liquid crystalline molecules, are used, the produced optical compensation sheet can have several optical properties corresponding to the displaying modes of the liquid crystal cell. The optical compensation sheets for several displaying modes, in which discotic liquid crystal molecules are used, are proposed in several patent applications. For example, Japanese Patent Laid Open Publication No. 6-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Application Publication No. 3911620A1 teach an optical compensation sheet for the TN-mode liquid crystal cell. Japanese Patent Laid-Open Publication No. 10-54982 teaches the optical compensation sheet for the IPS-mode or FLC-mode liquid crystal cell. Further, U.S. Pat. No. 5,805,253 and International Patent Application No. 96/37804 teaches the optical compensation sheet for the OCB-mode liquid crystal cell & the HAN-mode liquid crystal cell, Japanese Patent Laid-Open Publication No. 9-26572 teaches the optical compensation sheet for the STN-mode liquid crystal cell, and Japanese Patent No. 2866372 teaches the optical compensation sheet for the VA-mode liquid crystal cell.
However, although there are several ways of orientations of liquid crystal molecules, and only the above anisotropies of the liquid crystal sometimes cannot optically compensate the liquid crystal cell. U.S. Pat. No. 5,646,703 supposes that both the liquid crystal molecules and the transparent substrate of the optical compensation sheet is provided with the optical anisotropy. Thus the liquid crystal cell is optically compensated.
It is extremely progressed to provide several functions with the above products for several optical uses. In this progress of the products for the optical use (such as these LCDs), it is necessary for the film constructing each layer of the product to have the thickness uniformly controlled with high accuracy and the uniformity of optical properties. For example, the thickness nonuniformity (or thickness-mura) in the protective film for a polarizing filter causes wrinkle which occurs when being attached to the polarizing element, a curled shape of the polarizing filter after attachment, and the like. Further, the optical film is coated with a hard coating layer for surface protection and an anti-reflection layer for preventing the reflection. The thickness nonuniformity often causes the nonuniform coating (or coating-mura). Especially, when the nonuniformity (or mura) cyclically occurs, the quality of images displayed by the LCD becomes extremely worse. Each Japanese Patent Laid-Open Publication No. 2002-234042 No. 2002-1745 teaches a solution casting method as a method of producing a film, in which the thickness uniformity is improved moreover.
However, recently, while the thinner thickness and the larger number of superposed films and the improvement of productivity are increasingly required, it is necessary to control the thickness of the film produced from the polymer with higher accuracy. Accordingly, the productivity is not enough improved and the produced film cannot have the uniform thickness and the uniform optical property. Further, when the optical properties, especially retardation values Re, Rth in birefringence are not uniform, the influence on the products is large. For example, the tinting and the inversion sometimes occur in the LCD, and when such an optical compensation sheet for enlarging view angle is used in the LCD, the change of the view angle and the color inversion sometimes occur.